Maintaining a constant optical emission of laser diodes at logic level “0” or “P0” and logic level “1” or “P1” is a primary goal for optimal optical transmitter design to meet the requirements of high-speed fiber-optic applications. A proper laser bias current (corresponding to P0) must be established to minimize laser turn-on/turn-off delay caused by insufficient biasing. An appropriate bias current also limits the sensitivity penalty of optical receivers introduced by inadequate signal extinction ratio from over-biasing. A well-controlled laser modulation current (corresponding to P1) ensures meeting channel optical power budget without exceeding the optical receiver overload level Proper bias current and modulation current are accomplished by using an automatic power control (APC) loop and an automatic modulation control (AMC) loop. Alternatively, an extinction ratio control (ERC) loop can be used in place of an AMC loop.
For optical modules, it is desirable to integrate the laser diode and the driver in the same package or subassembly, since it is cost-effective for high-speed applications. Integration allows for minimizing the interface circuit between the laser and the driver for performance optimization. Under such circumstances, thermal management and space availability become critical for compact optical transmitter assemblies. This is due largely in part to the tight temperature range of operation for most lasers. As a result, a minimum driver die size is required to simplify the driver functionality for power saving requirements. Another requirement is that lasers may need a part-to-part adjustment for optimized bias and modulation currents. Therefore, it is beneficial to implement the APC and AMC loops separate from the laser-driver chip.
FIG. 1 is a block diagram of a prior art system 10 for controlling modulation and bias currents of a laser driver. System 10 has the APC and AMC loops incorporated into the pre-driver circuit, but are not shown. A laser diode 150, a laser driver 30 and a monitor diode 160 are assembled in a compact header 40. A high speed differential data signal is sent to the laser driver 30 at ports D+ and D−. The driver modulation and bias currents are set at Mod and Bias, respectively. The current from the monitor diode 160 is sensed at Monitor. Additionally, there are power supply (VCC) and ground (GND) ports. As can be seen, when the APC and AMC circuits are incorporated into the pre-driver 20, a total of seven pin connections are required to connect the pre-driver 20 to the header 40.
However, most low-cost/compact high-speed packages only have five pin connections, wherein the laser driver 30 and various other components reside in the header 40 of a package.
Accordingly, methods and techniques are needed to enable the APC and AMC to be incorporated into the pre-driver 30 and yet still utilize a low-cost/compact high-speed package with only 5 pins.